This invention relates to burners. In one aspect, the invention relates to flue gas recirculation (FGR) systems for burners while in another aspect, the invention relates to induced FGR systems for burners. In yet another aspect, the invention relates to induced FGR systems equipped with a fresh air purge apparatus, and the operation of such a system.
FGR systems are known, and U.S. Pat. No. 4,659,305 to Nelson and Rulseh provides a good description of such systems generally, and of forced FGR systems in association with a fire tube boiler, specifically. For example, Nelson and Rulseh describe the formation of nitrogen oxides during combustion, the interest in reducing nitrogen oxides emissions by the recirculation of flue gas, and various FGR systems that were known prior to the date of their invention. They then proceed to describe their invention, i.e. a forced FGR system which recovers a portion of the flue gas from a discharge stack and recirculates it by means of a recirculation fan to a point downstream of the fuel-air mixture in the burner. This results in a cooler flame which in turn substantially reduces the nitrogen oxides content in the boiler stack emissions. The burner assembly itself includes conventional damper and air diffuser systems and a plurality of openings near the inner end of the burner housing, through which the boiler fuel is emitted and mixed with air. Slots are formed at the outlet end of the burner, and are coupled to an annular chamber surrounding the burner. The annular chamber is connected to the recirculation fan by means of a duct.
While the forced FGR system of U.S. Pat. No. 4,659,305 performs admirably, it does require a recirculation fan, it does mix the flue gas with the combustion air downstream of the flue gas/air mixture, and it does use a flow control valve for the dual purposes of controlling the amount of recirculated gas admitted to the FGR system and to insure positive shutoff of gases that flow through the duct leading to the burner. Each of the features impact the overall capital cost and operating efficiency of the FGR system.
If the recirculation fan is eliminated from the FGR system, then the total electrical load needed to operate the system is reduced. In addition, the capital costs associated with acquiring and installing the fan are eliminated.
If the recirculated flue gas and combustion air are introduced prior to the combustion air fan and the burner diffuser, then the combustion air and recirculated flue gas are thoroughly mixed prior to entry into the combustion zone resulting in flame temperatures which are more homogeneous. This means that the number of hot spots and/or excess air pockets in which nitrogen oxides are typically formed are reduced, and this in turn means fewer nitrogen oxides emissions.
The addition of a fresh air purge to the FGR system can eliminate the need for a FGR shutoff valve. Moreover, a fresh air purge can provide an "air lock" which prevents combustible gases from flowing into the burner combustion air fan and combustion chamber from the FGR system until pilot ignition and main flame are safely established. In addition, the air purge motor starter can be electrically interlocked with the burner flame safeguard circuit providing a secondary proof that the FGR piping is safely purged.
These and other modifications to known FGR systems are more fully described below.